1. Field of the Invention
The present invention relates to a method of analyzing impurities existing in the surface of a semiconductor substrate, by using X rays, and more particularly to a method of analyzing aluminum existing in the surface of a silicon substrate, by using totally reflected fluorescent X rays.
2. Description of the Related Art
Hitherto, metal impurities in the surface of a silicon wafer have been analyzed by a so-called VPO-AA method, in which the metal impurities are dissolved with a hydrofluoric acid (HF) solution, and the metal impurities dissolved in the HF solution are analyzed by means of atomic absorption spectrometry. In this method, the HF solution is applied on the surface of the wafer, thereby collecting the impurities are collected from the surface of the wafer. With this method it is impossible to determine the plane distribution of the metal impurities.
A so-called TRXRF method is known, which is a non-destructive method of analyzing metal impurities in the surface of a silicon wafer. In the TRXRF method, fluorescent X rays are applied to the silicon wafer surface and the metal impurities are analyzed based on the X-rays reflected from the silicon wafer surface. With this method it is possible to determine plane impurity distribution in a spot size of about 10 mm.
FIG. 1 is a graph representing a distribution of metal impurities in the surface of a silicon wafer, which has been detected by means of the TRXRF method. Each metal impurity is identified by the energy which is indicated on the abscissa. The quantity of each metal impurity is determined from the strength of the reflected X rays which is plotted on the ordinate. The TRXRF method has a problem, however. Metals having almost the same energy can hardly be distinguished from one another. For example, silicon (Si) and aluminum (Al) have almost the same energy. The energy peak (1.49 keV) of Al overlaps that (1.74 keV) of silicon. Consequently, aluminum in the silicon wafer surface can hardly be detected. In brief, the TRXRF method can determine the distribution of metal impurities in a silicon wafer surface, but fails to identify the impurities with sufficient accuracy.